Hiding parts of mesh but only on defined objects

Hi,

I need to be able to hide parts of certain meshes in the scene by putting another mesh/object in front of them. Hopefully this illustration explains what I mean:

So there would be 2 objects with mesh, and only Mesh A would be hidden partially when the hider covers it.

I did find a shader that hides mesh that is behind it here http://answers.unity3d.com/questions/244634/hiding-parts-of-a-mesh.html. It works just fine, but the problem is it hides all mesh. Could it be possible to define it so that it hides only objects with a certain tag or something? Does shader code even have access to Unity tags? I have very limited experience on shaders, so help would be appreciated! =)